Piezoelectric transformer for fluorescent lamp

ABSTRACT

In the piezoelectric transformer for fluorescent lamp, the size of an electrode at the central region of each sides on the first face is smaller than that of electrode at the corner region. As a result, the stress generated in the piezoelectric block is minimized and the heat caused by the stress is prevented, so that the damage of the piezoelectric block and the deterioration of the efficient may be prevented. The input electrode can be formed in the various shapes such as diamond, cross, and rhombic shapes.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a piezoelectric transformer, andmore particular to the piezoelectric transformer for a fluorescent lampincluding a piezoelectric block having smaller electrodes at the centerregion of each sides than that at corner region to minimize the stressat the center region of the piezoelectric block in order to use forgeneral fluorescent lamp.

[0002] Since BaTiO₃ has been introduced at the mid-1940s as apiezoelectric seramic material, this seramic material having the goodpiezoelectric characteristics, such as Pb(Zr,Ti)O₃ (PZT), is useful forthe electric device. The PZT which is the solid solution of PbZrO₃ andPbTiO₃ is the 2-element based perovskite structure having the goodpiezoelectric characteristics. Recently, 3-element based perovskite hasbeen studied to vary easily the composition of the 2-element basedperovskite and to improve the piezoelectric characteristics. Forexample, Pb(Mg,Nb)O₃—Pb(Zr,Ti)O₃, Pb(Mg,Ta)O₃—Pb(Zr,Ti)O₃, andPb(Mn,Nb)O₃—Pb(Zr,Ti)O₃ are developed as the 3-element based complexperovskite compound.

[0003]FIG. 1 shows a applicable example of the piezoelectric material,i.e., a band pass filter for the amplitude modulation. Where, FIG. 1A isa plan view of the band pass filter and FIG. 1B is a sectional viewalong the line A-A of FIG. 1A. As shown in figures, the band pass filtercomprises a piezoelectric block of hexahedron shape having electrodes onthe upper and lower faces. At this time, the electrodes on the upperface are input/output electrodes 3 and 5 and the electrode on lower faceis a common electrode 7.

[0004] When the AC voltage corresponding to the resonant frequency issupplied to the input electrode 3, the electric signal is converted intothe strong mechanical vibration near the input electrode and theconverted vibration is transmitted to the output electrode. As a result,the voltage proportional to the resonant frequency is outputted from theoutput electrode 5.

[0005] The piezoelectric material has been introduced at end-1910s andapplied to the various electronic device such as a high voltagegenerator, an ultrasonic generator, sound facilities, a 45.5 KHz IFfilter for AM radio and a 10.7 MHz filter for FM radio, a communicationdevice, and the various sensors. Further, this piezoelectric materialhas been used for a resonator or filter for communication device, aninverter of a cold cathode tube for a backlight of a liquid crystaldisplay. In addition, the piezoelectric transformer used for the generalinverter is also introduced.

[0006] The structure of the typical transformer composed with thepiezoelectric material is shown in FIG. 2. Where, FIG. 2A is aperspective view and FIG. 2B is a sectional view. This transformer isRosen type transformer having vibrations mode in the thickness andlength directions. As shown in figures, the transformer is polarized inthe thickness direction at the input electrode region on a part of theupper and lower faces. In the output electrode region, further, it ispolarized in the length direction. When the AC voltage corresponding tothe resonant frequency is supplied to the input electrodes 3 a and 3 bto raise the voltage of the piezoelectric transformer, the suppliedelectric signal is converted into the strong mechanical vibration of thelength direction near the input electrodes and then transmitted to theoutput electrode to generate the longitudinal vibration. Then thelongitudinal mechanical vibration is outputted from the output electrode5 as the raised high voltage having the high frequency that isidentified with the frequency of the voltage supplied to the inputelectrode.

[0007] At this time, the raise of the voltage at the output electrode ismaximized when the frequency of the input voltage is identified with themechanical vibration frequency at the output electrode. Further, thevoltage-raise ratio of the piezoelectric transformer, which is dependantupon the load impedance of the material, is maximized when therelatively higher load impedance is applied to the output electrode. Incase of the lower load impedance, the voltage raise ratio is less thanseveral tens times.

[0008] When the piezoelectric transformer is used for the lamp such asthe cold cathode tube and the fluorescent lamp, it has the differentload impedance in accordance with the kind of the lamp. However, if thepiezoelectric is made under the optimum fabrication condition, the highvoltage-raise ratio can be maintained under the condition of the lowerload impedance. In the normal state that the impedance is decreasedafter lighting, further, the voltage-raise ratio can suitably bemaintained so that the piezoelectric transformer can be used for thelamp such as the cold cathode tube and the fluorescent lamp.

[0009] Recently, the piezoelectric transformer having the outlinevibration mode shown in FIG. 3 is also introduced. As shown in FIG. 3,the piezoelectric transformer of the outline vibration mode has thestructure similar with that of the band pass filter shown in FIG. 1except the shape of the electrodes. That is, the electrode in FIG. 3 isthe circular shape, while the rectangular shape in FIG. 1. In thispiezoelectric transformer, the output electrode 5 is disposed on theupper face of the piezoelectric block 1 at the predetermined distancefrom the input electrode 3 and the common electrode 7 is disposed on thelower face of the piezoelectric block 1.

[0010] When the voltage is supplied to the piezoelectric block 101through the input electrode 3, the electric signal is converted into themechanical vibration directed to the side portion from the centralportion of the piezoelectric block 1 and then the signal proportional tothe mechanical vibration is outputted though the output electrode 5.This piezoelectric transformer may be used for the liquid crystaldisplay of notebook computer and the low-consumption transformer.

[0011] There, however, is a problem that the piezoelectric material isnot applicable to the high power transformer. Since the Rosen or thedeposited Rosen type piezoelectric transformer (U.S Pat. No. 6,037,706)has the complex structure and outputs the low output signal, it is notapplicable to the fluorescent lamp. In the outline vibrationmode-piezoelectric transformer, the deposited structure of thepiezoelectric material should be studied because of the problem of thematerial. The electrodes of the circular and rectangular shapes in theoutline vibration mode-piezoelectric transformer is disclosed in ‘Designof Fluorescent lamp with PFC using a power piezoelectric transformer,Sung Jin Choi, IEEE(1998. 2. 15), P1141’. In this transformer, however,there is a problem that the out signal is low.

[0012] Further, there is a problem that the stress is maximized in apart of the piezoelectric block so that the piezoelectric device may bedestroyed and the efficiency thereof may be deteriorated, when theelectric signal is converted into the mechanical vibration.

SUMMARY OF THE INVENTION

[0013] It is an object of the present invention to provide apiezoelectric transformer in which the electrode at the low vibrationregion of the piezoelectric block, the central region of the sides, isminimized to eliminate the heat caused by the stress and prevent thedamage of the piezoelectric block.

[0014] In order to achieve the object, in the piezoelectric transformerof the present invention the size of the electrode at the stressgeneration region on the upper face of the piezoelectric block isdecreased to minimize the heat caused by the stress. The stress ischiefly generated near the central region of each sides of thepiezoelectric block. By minimizing the size of the electrode at thisregion, thus, the stress is also minimized and as a result thispiezoelectric transformer is applicable to the high power transformer.For the minimized electrode, the electrode can be formed in the diamondshape or the cross shape. The shape of the electrode can be formed inthe various shapes, without the limitation.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a view showing the structure of the conventional bandpass filter using the piezoelectric material.

[0016]FIG. 2 is a view showing the structure of the conventionalpiezoelectric transformer of Rosen type using the piezoelectricmaterial.

[0017]FIG. 3 is a view showing the structure of the conventionalpiezoelectric transformer of outline vibration mode using thepiezoelectric material.

[0018]FIG. 4 is a view showing the structure of the piezoelectrictransformer for the fluorescent lamp according to one embodiment of thepresent invention.

[0019]FIG. 5 is a view showing the mechanical vibration when the voltageis supplied to the piezoelectric transformer for the fluorescent lamp.

[0020]FIG. 6 is a view showing the structure of the piezoelectrictransformer for the fluorescent lamp according to other embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0021] In general, the back light used for the liquid crystal displaydevice has the Rosen type-piezoelectric transformer. This Rosen typepiezoelectric transformer used as the lighting device of the coldcathode tube has the high voltage and low current characteristics sothat it is not applicable to the general fluorescent lamp. Duringlighting, the impedance of the general fluorescent lamp is less than fewKΩ, while the impedance of the cold cathode tube is approximately80-100KΩ. Thus, the impedance of the general fluorescent lamp is farless than that of the cold cathode tube.

[0022] This invention provides the piezoelectric transformer applicableto the general fluorescent lamp. To fabricate this piezoelectrictransformer, in this invention, the electrodes have the differentstructure from that of the conventional piezoelectric transformer. Bythis electrode structure, the maximum stress is generated at thedifferent region from that of conventional piezoelectric transformer andas a result the desired current and voltage may be obtained.

[0023] Hereinafter, the piezoelectric transformer according to thepresent invention will be described in detail, accompanying drawings.

[0024] FIGS. 4 are view showing the piezoelectric transformer of thepresent invention. Where FIG. 4A is a plan view and FIG. 4B is asectional view. As shown in figures, the piezoelectric transformer ofthe present invention comprises the piezoelectric block 101 having thehexahedral shape, the input/output electrodes 103 and 105 on the upperface of the piezoelectric block 101, and the common electrode 107 on thelower face of the piezoelectric block 101.

[0025] The input electrode 103 is substantially formed in the rhombicshape or the diamond shape and separated from the output electrode 105in the predetermined distance. The common electrode 107 is integrallyformed on at least a part of the lower face of the piezoelectric block101.

[0026] In this piezoelectric transformer, when the electric signal isinputted through the input electrode 103 and the common electrode 107,the electric signal is converted into the mechanical vibration of theoutline vibration mode within the piezoelectric block 101 and then thesignal proportional to the mechanical vibration is outputted through theoutput electrode 105.

[0027] As shown the dash double-dot line of FIG. 5, when the voltage issupplied, the resultant vibration is maximized at the corner region ofthe piezoelectric block, while minimized at the central region of eachsides. In the figure, an arrow in the corner region indicates the degreeof the vibration in the piezoelectric block 101. At this time, thedegree of the vibration is dependant upon the piezoelectric material andthe supplied voltage.

[0028] Since the mechanical vibration is maximized at the corner regionof the piezoelectric block 101 and minimized at the central region(indicated as P in the figure) of the sides, as shown above, the maximumstress is acted at the central region of the piezoelectric block 101 andthe second high stress is acted at the central region of the sides ofthe piezoelectric block 101. By this stress, therefore, the heat isgenerated at the central region of the sides, so that the piezoelectricblock may be damaged.

[0029] When the electric signal is converted into the mechanicalvibration, the vibration is strongly generated at the input/outputelectrodes. Thus, if the size of the electrode at the central region Pof the sides is decreased, the degree of the vibration is alsodecreased. As a result, the less heat is generated at this region. Atthis time, the size ratio of the input electrode 103 and the outputelectrode 105 is preferably 1:1.5-1:3.14.

[0030] In order to form the electrode having above size ratio, the inputelectrode 103 is formed in the diamond shape. Thus, the area between theinput and output electrodes 103 and 105 where the electrode is notdisposed is extended near the central region P of the sides of thepiezoelectric block 101 so that the size of the electrode is decrease atthe region P.

[0031] In case where the input electrode 103 is formed in the diamondshape as shown above, the energy conversion efficiency is improved andthe capacitance is increased to decrease the impedance at the output,because the electro-mechanics coupling coefficient Kp in the radiusdirection is higher than the coupling coefficient K31 in the lengthdirection. As a result, the output power may be increased. Further, theraise ratio of the voltage is decreased so that this piezoelectrictransformer is applicable to the lighting of the fluorescent lamp havingthe low impedance in case of the input electrode 103 of the diamondshape.

[0032] It is not necessary to limit the shape of the input electrode 103as the diamond shape. In other word, the electrode can be formed in anyshape that the stress can be decreased in accordance with the decreaseits size at the central region P of the sides of the piezoelectric block101 to minimize the heat generation.

[0033] As shown in FIG. 4B, the common electrode 107 is integrallyformed at a part on the lower surface of the piezoelectric block 101 andthe input/output electrodes 103 and 105 are connected to the outercircuit (not shown in figure). In this case, the noise may be inputtedfrom the outer circuit through the common electrode 107. In order tosolve this problem, the common electrode 107 can be made in a pluralityof isolated electrodes as shown in FIG. 6A. By this isolation of thecommon electrode 107, the noise cannot be inputted to the piezoelectrictransformer from the outer circuit. In general, the common electrode 207is formed in the different shape from those of the input electrode 203and the output electrode 205, as shown in FIG. 6A. However, the commonelectrode 207 may be formed in the same shape with the input electrode203 and the output electrode 205 as shown in FIG. 6B.

[0034] In the piezoelectric transformer of the present invention, asdescribed above, the electrode at the central region of each sides ofthe piezoelectric block has smaller size than that of the electrode atthe corner region. At this time, the shape of the electrode is notnecessary to be specified. The input electrode can be formed in thevarious shapes such as the diamond, rhombic, or cross shapes.

[0035] As described above, the size of the electrode is minimized at themaximum stress generating area to decrease the stress. Thus, when thepiezoelectric transformer is used for the high voltage transformer, thedamage of the piezoelectric block or deterioration of the efficiencycaused by the heat can be prevented.

[0036] While the invention has been described in its preferredembodiments, this should not be construed as limitation on the scope ofthe present invention. Accordingly, the scope of the present inventionshould be determined not by the embodiments illustrated, but by theappended claims and their legal equivalents.

What is claimed is:
 1. A piezoelectric transformer comprising: apiezoelectric block having first and second faces to convert electricsignal into mechanical vibration by outline vibration mode; an inputelectrode at the central region on the first face, the electric signalbeing inputted to the input electrode; an output electrode at theperipheral region on the first face to convert the mechanical vibrationinto the electric signal and output the converted electric signal, theoutput electrode being separated from the input electrode inpredetermined distance; and a common electrode on the second face of thepiezoelectric block; wherein the size of the input and output electrodesis minimized at the central region of the sides of the first face. 2.The piezoelectric transformer according to claim 1 , wherein the inputelectrode includes diamond shape.
 3. The piezo electric transformeraccording to claim 2 , wherein the size ratio of the input and outputelectrodes is approximately 1:1.5-1:3.14.
 4. The piezoelectrictransformer according to claim 1 , wherein the common electrode isintegrally formed on at least a part of the second face of thepiezoelectric block.
 5. The piezoelectric transformer according to claim1 , wherein the common electrode is formed on the second face of thepiezoelectric block in a plurality of isolations to prevent the input ofthe noise.
 6. The piezoelectric transformer according to claim 1 ,wherein the common electrode is facing with the input and outputelectrodes.
 7. The piezoelectric transformer according to claim 1 ,wherein the input electrode includes diamond shape and the outputelectrode at the central region of the sides on the first face of thepiezoelectric block has smaller size than that of the output electrodeat the corner region.
 8. The piezoelectric transformer according toclaim 1 , wherein the input electrode is substantially cross shape.
 9. Apiezoelectric transformer comprising: a piezoelectric block having firstand second faces to convert electric signal into mechanical vibration byoutline vibration mode; an input electrode disposed closer to thecentral region of each sides than to the corner region on the first faceof the piezoelectric block, the electric signal being inputted to theinput electrode; an output electrode separated from the input electrodein predetermined distance at the peripheral region on the first face sothat its size at the central region of each sides is smaller than thatat the corner region, the output electrode converting the mechanicalvibration into the electric signal and outputting the converted electricsignal; and a common electrode on the second face of the piezoelectricblock.